I really don't see the issue here. SHACL, like ShEx is a language to express data shapes. I adopted using ShEx in a wikidata context 2016 when ShEx was demonstrated at a tutorial at the SWAT4HCLS conference [1] in Amsterdam, where it was discussed in both a tutorial and a hackathon topic. At that conferene, I was convinced that ShEx is helpful in maintaining quality in Wikidata. ShEx offers not only the means to validate data shapes in Wikidata, but it also provides a way to document how primary data is expressed in Wikidata. In 2016 I joined the ShEx community group [2]. Since I have been actively using ShEx in defining shapes in various projects on Wikidata (e.g. Gene Wiki and Wikicite). It is not that this happened in secrecy. On the contrary, it was discussed at both Wikimedia [3,4] and non-Wikimedia events [5,6,7].
It is also not the case that SHACL has not been discussed in this context, on the contrary, I have very good memories of a workshop where both were debated (see page 24 ;) ) [8]
IMHO the statement that we all should adhere to one standard, simply because it is a standard, is not a valid argument. Imagine having to dictate that we all should speak English because it is the standard language. In every single talk that I have given since 2016, proponents of SHACL have been very vocal in asking the same question over and over again "why not SHACL?", where the discussion never went beyond, "You should because it is a standard". It is also a bit disingenuous to suggest we all should adhere to SHACL because it is the standard, while in the same sentence calling it a "Recommendation".
Although initially, I was open to SHACL as well (I use both Mac and Linux, so why not open up to different alternatives in data shapes), (Some) Arguments for me to prefer ShEx over SHACL are:
1. Already in 2017 there were different (open) implementations. At the time SHACL didn't have much tooling to choose from, other than one javascript implementation and a proprietary software package.
2. ShEx has a more intuitive way of describing Shapes, which is the compact syntax (ShExC). SHACL seems to have adopted the compact syntax as well, but only yesterday [9].
3. The culture in the Shape Expression community group aligns well with the culture in Wikidata.
4. I don't want to be shackled to one standard (pun intended). I assume the name was chosen with a shackle in mind, which puts constraints at the core of the language. Wikidata already has different methods in place to deal with constraints and constraint violations. In the context of Wikidata, ShEx should specifically not be intended to impose constraints, on the contrary, it allows expressing of disagreement or variants of different shapes, whether conflict or not. Which fits well with the NPOV concept. Symbols do matter.
For a less personal comparison, I refer to the "Validating RDF data" book which describes both ShEx and SHACL, and has a specific chapter on how they compare and differ [10]
Up until now, I have been using ShEx in repositories outside the Wikidata ecosystem (e.g. Github), but I am really excited about the release of this extension. I am curious about how the wiki extension will influence the maintenance of schemas. Schemas are currently often expressed as static images, while in practice the schemas are as fluid as the underlying data itself. Being able to document these changes dynamically (the wiki way), can be very interesting. One specific expectation I have is that it might make it easier to write federated SPARQL queries. Currently, when writing these federated queries we often have to rely on either a set of example queries or a one-time schema description, which makes it hard to write those queries, because of schemas changing constantly. Federated SPARQL queries now really is a process of "slot machine" querying, where one has to explore the underlying schema, query by query. With a wiki in place and a community maintaining these ever-changing schema's, I expect better documentation.
The data shape community, instead of adhering to one language, should really be proud to have produced two very helpful languages. ShEx and SHACL are similar but do have differences so both have merit to exist and I wish we could steer away from this ShEx vs SHACL feud. It really isn't helping the cause, i.e. being able to express schemas in a formal language. Honestly, this fued really reminds me of the famous monty python sketch, "The machine that says Bing". Let us focus on the patient and not on the "Bing".
Just my 2ct.